This annex defines a set of conventions for writing code in which an instance of a subprogram executes on only one processor (of which there may be more than one).
If a program unit has extrinsic kind HPF_SERIAL, an HPF compiler should assume that the subprogram is coded to be executed on a single processor. From the point of view of a global HPF caller, the HPF_SERIAL procedure behaves the same as an identically coded HPF procedure would. Differences might only arise in implementation-specific behavior (such as the performance).
The EXTRINSIC mechanism, which allows an HPF programmer
to declare a calling interface to a non-HPF subprogram,
is described in Section 
of the HPF specification.